Thursday, 13 February 2003
The role of vortex mergers in the genesis of tropical cyclones
Recently Simpson et al. (1997) have proposed that tropical cyclogenesis occurs as a result of mesoscale vortex interaction involving mesoscale convective systems that leads to vortex merger growth to the surface and subsequent development of both the nascent eye and inner rainbands of a tropical cyclone. This paper inspired the study by Karyampudi and Pierce (2002; referred hereafter as KP), who showed that vortex mergers play an important role in the large-scale tropical cyclogenesis as well. KP study also validated the baroclinic mechanism proposed by Karyampudi and Carlson (1988), which hypothesizes that tropical cyclogenesis over the E. Atlantic is influenced by the dynamical forcing exerted by the Saharan Air Layer (SAL) on African wave disturbances. However, KP showed that the baroclinic mechanism is inadequate in explaining the deep vortex formation as it relied on CISK mechanism for vortex growth. Instead, they found that merger of low and mid-level vortices dominated the deep vortex formation in all the three cases they studied using the large-scale ECMWF data. In a follow-up study, Karyampudi and Davison (2001) have further shown from a preliminary investigation of all the tropical storms formed in the Year 2000 Atlantic Hurricane season that indeed a majority of the tropical storms formed due to the merger of mid-tropospheric and low-level vortices. Such mid-level and low-level large-scale vortex mergers in the formation of tropical cyclones is consistent with the study reported by Molinari et al. (2000). Karyampudi and Davison also found that the availability of low-level vortices within the monsoon environment is in agreement with the genesis study over the Western Pacific reported by Simpson et al. (1997), who claimed that “monsoon trough provides a region of reduced deformation radius, which subsequently improves the efficiency of mesoscale vortex interactions and the amplitude of merged vortices.” Thus, it appears that vortex mergers are an essential component of tropical cyclogenesis at various stages of development. A review of both the mesoscale and large-scale vortex merger studies involving tropical cyclone genesis will be presented at the conference to pay a tribute to the important contributions made by Dr. Joanne Simpson and her colleagues in understanding the scale interaction processes involved in the genesis of tropical cyclones.